1. Field of the Invention
The present invention relates to an optical glass and more particularly to an optical glass of high refractive index and high dispersion having a refractive index nd of 1.70 or more and an Abbe's number .nu.d of 30 or less.
2. Description of Prior Art
As such an optical glass, there are disclosed, for example, a glass comprising, as main components, P.sub.2 O.sub.5, Na.sub.2 O, TiO.sub.2 and WO.sub.3, in Japanese Patent Application Kokai (Laid-Open) No. 17409/1974, a glass consisting of a P.sub.2 O.sub.5 -R.sub.2 O (R is a mono-valent metal)-ZnO-Nb.sub.2 O.sub.5 type glass, in Japanese Pat. Publication No. 40094/1981, and a glass in which BPO.sub.4 and/or Al(PO.sub.3).sub.3 is incorporated into a SiO.sub.2 -TiO.sub.2 -Nb.sub.2 O.sub.5 -R.sub.2 O (R is a mono-valent metal) type glass, in Japanese Pat. Publication No. 54247/1985.
However, the glasses disclosed in Japanese Pat. Application Kokai (Laid-Open) No. 17409/1974 and Japanese Pat. Publication No. 40094/1981 have low devitrification resistance. The glass disclosed in Japanese Pat. Publication No. 54247/1985, when the refractive index nd is increased to 1.70 or more, has a high devitrification tendency in the vicinity of the softening temperature and accordingly is devitrified when the glass block is heated and softened, making the press molding difficult.
Hence, an object of the present invention is to provide a high refractive index and high dispersion optical glass which has a refractive index nd of 1.70 or more and an Abbe's number .nu.d of 30 or less and which is highly resistant to devitrification and stable.
Other objects of the present invention are apparent from the following descriptions.